It is known that, under some operating conditions, the stoppage of a turbine engine such as a turbojet or a turbopropeller can result in a turbine mechanical lock, due to a differential expansion between stator and rotor elements. This phenomenon is known as “core lock” or “rotor lock”. This lock makes it impossible to re-start the engine: upon a flight, in the case of an abrupt shutdown, but also on the ground, in the case of too an early shutdown after landing or for fuel saving purposes, running is sometimes performed with a reduced number of engines.
Beyond the “core-lock” effect, the present invention is also of interest as regards protection against thermomechanical fatigue effects. When subjected to intensive and frequent thermal cycles, metal parts of the engine components actually undergo thermomechanical type stresses which may lead to cracks in the metal structure of the engine components. Cracks initiated within the compressor or turbine disks for example can propagate and result in the breaking thereof. The high speeds of rotation of these elements, as well as their big size, can result upon breaking thereof in significant damage to the propulsive system, to the aircraft or to the environment, because the energy level being induced is then very high.
The engine shutdown phases represent significant exposures to the thermomechanical fatigue phenomenon, if engine shutdown precautions, consisting in waiting several minutes with an engine maintained at an idle speed before shutting down, are not applied.
In the case of the engine being stopped by an abrupt shutdown, for example caused by an aerodynamic distortion, a temporary fuel deprivation or on a crew command, the air used for cooling mechanic parts of the turbine does not flow any longer sufficiently within the engine. If the aircraft travel speed is low, the “windmilling” engine driving (engine rotation under the effect of the air speed penetrating it, by a “windmill” effect) engine driving is too low or absent. Then, the differential expansion and contraction between the turbine rotor, stator and casing are such that these parts can come into mechanical contact and can make it impossible to re-start the windmilling engine, or even with the assistance of the starter. The differential heat expansion and contraction can have effects in the radial/longitudinal direction. If there is no contact and no rotation possible, re-starting an engine which has been shut down without waiting for a sufficiently long period at idle can result in vibrations or frictions at the end of the rotating parts, generated for example by a so-called “rotor bow” phenomenon where the shaft bearing the compressor stages and the turbine as well as the fixed structure consisting of the stators and the casing are bowed between the two ends thereof. Starting under an excessive “rotor bow” and a contact with the compressor vanes can initiate cracks in the latter.